The high prevalence of pediatric overweight and obesity in the U.S. necessitates the development of efficacious, but cost-effective and easily disseminable interventions that improve and sustain better pediatric weight outcomes. Accumulated evidence finds initial and long-term efficacy of moderate-to-high intensity (>25 contact hours) family-based treatment (FBT) for elementary school-aged children, but such treatment is prohibitively expensive because of high personnel costs (delivery by behavioral health professionals) and therefore limited in availability, reach, and impact. Very few overweight/obese children receive evidence-based treatment to improve their weight status or health. Our pilot data suggest a new model, peer-based delivery FBT with FBT-treated parents trained and subsequently delivering FBT to other families, is feasible and demonstrates initial efficacy. This delivery model could dramatically reduce FBT costs and increase FBT availability. Preliminary data also suggest that serving as a peer interventionist may also benefit the peers themselves and their children in better sustaining their own weight outcomes. The present study aims to provide a more definitive test of the short- and long-term efficacy of peer- versus professionally-delivered FBT on child weight outcomes. Embedded in this comparison is an examination of the feasibility of peer-based FBT delivery, investigating the impact of serving as a peer interventionist on the peers' own and their child's long- term weight status, and a more comprehensive examination of differential costs of professional- versus peer- based FBT delivery. The present study will also test potential dissemination and sustainability by examining the efficacy of peer-delivered FBT among peer interventionists who themselves received FBT from other peers and not from professional interventionists. Moderation of treatment outcome is also examined, focusing on peer interventionist and child characteristics. This multi-phase trial will enroll 304 families with 7-11 year old overweight/obese children in multiple sites throughout our region, and include short-term (post-treatment) and long-term (up through 1 year after treatment cessation) assessments. This investigation of an innovative approach that could significantly reduce FBT delivery costs, while also improving the maintenance of treatment effects, is critical to improving the health of the many already overweight children at risk for chronic disease.